In integrated semiconductor memories, digital information items are stored in a multiplicity of memory cells, which are in each case connected to a bit line and to a word line. Volatile semiconductor memories, in particular DRAMs (dynamic random access memory), have memory cells each having a selection transistor and a storage capacitor. The storage capacitor may be formed as a trench capacitor or as a stacked capacitor, and the selection transistor may be formed in particular as a MOSFET (metal oxide semiconductor field effect transistor). One electrode of the storage capacitor is connected to one source/drain region of the selection transistor. The other source/drain region is connected to a bit line. The gate electrode is connected to the word line and at the same time forms a word line section. The gate electrode is isolated from a channel region of the selection transistor by a gate oxide. A multiplicity of selection transistors are connected to each word line.
Present-day semiconductor memories generally have segmented word lines each having a master word line and a multiplicity of word line segments connected to the master word line. The word line segments are usually formed from polysilicon and form the gate electrodes of the connected selection transistors. In general, the same number of selection transistors is connected to each word line segment of a word line. The metallic master word line has a higher electrical conductivity than the word line segments made of polysilicon. In order that the master word line can be dimensioned wider for the sake of further improved conductivity, it is often connected to a plurality of rows of word line segments, for instance two rows of word line segments running alongside one another. It is thereby possible to reduce the number of master word lines in relation to the number of word line segments.
The metallic master word line serves for rapidly transmitting the word line potentials to all the connected selection transistors. On account of the increasing miniaturization of structure elements of modern semiconductor circuits and on account of the ever higher transmission speeds and clock frequencies, the word line segments are nowadays driven actively in order to achieve faster switching of the selection transistors connected to the word lines. In this case, in addition to a word line driver to which the master word line is connected, a driver segment is also provided for each word line element, which driver segment supplies the respective word line segment (and also in each case a word line segment of further adjacent word lines) directly with the respective word line potential. From the multiplicity of driver segments, the word line potential passes to all the connected selection transistors more rapidly than if only the end of the master word line is connected to the word line potential. The potential provided for activating a word line is usually designated by Vpp, and the potential provided for deactivating the word line is designated by Vnwll. If a segmented word line is driven actively, these two potentials are in each case applied directly to all the word line segments of the word line to be activated or deactivated.
On account of manufacturing tolerances, the electrical connections between the word line segments and the master word line may be at high impedance or completely interrupted so that an individual word line segment cannot be brought or cannot be brought rapidly enough to the envisaged word line potential. Defective electrical connections may arise, for example, as a result of high-impedance contact hole fillings that connect a word line segment to the master line. In the case of an actively driven word line, there are even two contact hole fillings provided between a word line segment and the master word line. Within the driver segments of the word line driver as well, defective or high-impedance electrical connections such as contact hole fillings may prevent the word line segment that is to be driven from being supplied with the respective word line potential.
By way of example, it may happen that on account of defective contact hole fillings within a driver segment, although the word line segment can be activated, it cannot be deactivated, or vice versa. Such circuit faults are based on production tolerances, particularly in the lithographic patterning of contact holes, if, during the production of etching masks, for example, lateral positional errors occur, as a result of which conductive structures that are to be arranged on one another are not connected to one another at sufficiently low impedance. Furthermore, surface alterations of conductive structures produced, contaminants or other influences may lead to electrical decoupling of individual word line segments. Such word line segments that are not supplied, or are not supplied rapidly enough, with the respective word line potential are influenced by electrical potentials in their vicinity after the end of the activation operation or deactivation operation and assume a fluctuating or “floating” potential, which adapts to the respective ambient potential and is virtually unforeseeable. The selection transistors connected to these floating word line segments then switch uncontrollably, as a result of which the charges stored in the connected storage capacitors may pass on to the bit lines. This leads to read-out errors when reading from other, actually intact memory cells. On the basis of a test pattern of tested memory cells, it is thus not possible to reliably localize which word line segments are floating.
The localization of floating word lines is furthermore made more difficult, in the case of segmented word lines driven segment by segment with the aid of driver segments, when only one of the word line potentials Vpp and Vnwll to be provided is not fed, or is not fed rapidly enough, to the word line segment. This is caused by the design of the driver segments, which have an inverter having a pFET transistor and an nFET transistor, of which one transistor is connected to the potential Vpp and the other transistor is connected to the potential Vnwll. The potential fed to the master word line switches one of the two transistors into the on state, so that the connected word line segment ideally immediately assumes the envisaged potential. However, if one of the two transistors has no or only a high-impedance electrical connection to the word line segment, either only the activation or the deactivation of the word line segment functions.
Semiconductor memories having word lines which, although having been replaced by redundant word lines and permanently electrically disconnected from the voltage supply, are nevertheless floating cannot be reliably used and have to be disposed of as rejects. At least when a defective word line cannot be deactivated and only its activation operation functions, the potential of the disconnected, but nevertheless floating word line tends toward the activation potential Vpp and leads to unforeseeable read-out errors when reading from other memory cells not connected to the relevant word line at all. Since, in this case, replacing the word line by a redundant word line does not prevent the defective word line from floating in a manner tending toward the activation potential, the relevant semiconductor chip is not functional even after a word line repair.
If, conversely, a word line can indeed still be deactivated, but can no longer be activated (for instance in the case of an intact connection to the deactivation potential Vnwll and a defective connection to the activation potential Vpp), after the defective word line has been replaced by a redundant word line, the semiconductor chip can continue to be used, by contrast, since the floating potential—tending toward the Vnwll—of the replaced word line keeps the selection transistors of the connected memory cells closed. For distinguishing the case of whether or not the semiconductor chip can still be used after a defective word line has been replaced, it is necessary to carry out a complicated electrical functional test, to be precise after repairing and replacing defective word lines. Since such a functional test has already been carried out in order to identify defective word lines, the functional test carried out subsequently increases the manufacturing costs still further.
For possible use of the semiconductor chip, it must nevertheless be ensured that no read-out errors occur on account of floating word line segments. If it were possible to distinguish on the basis of an electrical functional test whether the potential of a floating word line tends toward the activation potential or toward the deactivation potential, the reject rate of the fabricated semiconductor chips could be significantly reduced. The semiconductor chips whose floating word lines tend toward the deactivation potential after they have been replaced by redundant word lines could continue to be used, whereas those semiconductor chips whose word line potential of the affected word lines tends toward the activation potential would be disposed of. However, in order to make a reliable ascertainment about whether the potential of a floating, replaced word line tends toward the activation potential or toward the deactivation potential, it is necessary to carry out electrical functional tests which, if the intention is to make an ascertainment that is probably if not certainly accurate, requires such a long expenditure of time that carrying out such a test is no longer economically practical. For this reason, this additional functional test is often dispensed with in practice.